1. Field of the Invention
The invention relates generally to cellphone GPS location systems and more particularly to a positioning system using radio signal times-of-arrival for determining a radio positioning system (RPS)-based position and time and then using the RPS-based position and time for improving the speed and sensitivity of a GPS receiver for acquiring GPS signals.
2. Description of the Background Art
The global positioning system (GPS) is a system using GPS satellites for broadcasting GPS signals having information for determining position and time. Each GPS satellite broadcasts a GPS signal having twenty millisecond data bits modulated by a one millisecond pseudorandom noise (PRN) code having 1023 bits or chips. The PRN code is known as a spreading code because it spreads the frequency spectrum of the GPS signal. This spread spectrum signal is known as a direct sequence spread spectrum (DSSS) signal.
The GPS signals from all the satellites are broadcast at about the same carrier frequency. However, the PRN code for each GPS satellite is different, thereby enabling a GPS receiver to distinguish the GPS signal from one GPS satellite from the GPS signal from another GPS satellite. The twenty millisecond data bits are organized into frames of fifteen hundred bits. Each frame is subdivided into five subframes of three hundred bits each. Near the start of each subframe is a Zcount having the GPS clock time that the subframe was transmitted. A GPS receiver determines a one-way range, called a pseudorange because it includes a local time offset, to a GPS satellite from the time-of-arrival of the PRN code, the Zcount and ephemeris parameters in the GPS signal that it receives from that GPS satellite. Normally four or more pseudoranges are used for determining or overdetermining a three dimensional position and GPS time. Three GPS satellites are sufficient if altitude is known or assumed.
GPS receivers are used in many applications where it is important to minimize the time duration between the time when the GPS receiver is turned on, or returns from a standby mode, and the time when it provides its position. This time duration, known as the time to first fix (TTFF), generally includes (i) the time to acquire GPS signal power by tuning a local frequency and a local PRN replica code phase in the GPS receiver to match the carrier frequency and the PRN code phase of the incoming GPS signal, (ii) the time to receive data bits in the GPS signal to determine a GPS clock time, (iii) the time to receive ephemeris parameters in the GPS data bits, and (iv) the time to process the code phase timing, GPS clock time and ephemeris for determining a position.
Conventional GPS receivers acquire signal power with a search algorithm. In a typical search algorithm, the local frequency is set to a first trial frequency and then correlations are determined between the incoming GPS signal PRN code and all possible code phases of a local replica of the code. In order ensure that the correct code phase is not missed, it is conventional to increment the replica code phase in one-half chip or even smaller steps. A high correlation value indicates that signal power has been found. If no correlations are high enough, the local frequency is set to a second trial frequency and the correlations are repeated. Although no one correlation will take a great deal of time, the great number of correlations that must be performed can result in the time to find signal power to acquire a GPS signal being the largest single component of the time to first fix (TTFF).
There is a need for reducing the time that is required to acquire signal power in a GPS signal in order to reduce the total time to first fix in a GPS receiver.